1. Field of the Invention
This invention relates generally to closed-loop control for active vehicle wheel steering and, more particularly, to closed-loop control for active front wheel steering or active rear wheel steering, where the closed-loop control uses longitudinal wheel slip to improve the vehicle's directional stability.
2. Discussion of the Related Art
It is known in the art to employ automatic rear-wheel vehicle steering based on vehicle dynamic information during a vehicle turn, or yaw, to enhance the vehicle stability. Active wheel steering control of a vehicle can improve vehicle stability over a conventional vehicle with only two steerable front wheels. The rear wheel steering assist can be in-phase steering or out-of-phase steering. In-phase rear wheel steering steers the rear wheels in the same direction as the front wheels, and is typically provided at higher vehicle speeds. Out-of-phase rear wheel steering steers the rear wheels in an opposite direction as the front wheels to provide a tighter turning radius, and is typically provided at lower vehicle speeds.
Open-loop automatic rear-wheel steering provides a certain amount of rear-wheel steering assist depending on the amount of front-wheel steering provided by the vehicle operator. In other words, a predetermined function is used to turn the rear wheels a certain amount in response to the turning of the front wheels at a particular vehicle speed. It is known to provide closed-loop automatic rear-wheel steering based on feedback in the event that the vehicle is not following the steering path requested by the vehicle operator. For example, slippery road conditions may prevent the vehicle from turning in the desired direction because the wheels may slip along the road surface. Further, the vehicle's rear quarter may “fish-tail,” also providing a different turn angle than was intended. Closed-loop rear-wheel steering assist systems sense the actual vehicle yaw rate and the intended yaw rate, and generate a gain signal that provides the steering assist by the rear wheels if the vehicle yaw rate and the intended yaw rate are not the same.
When the vehicle wheels are turned, the lateral force between the roadway and the vehicle wheels causes the vehicle to turn. The greater the lateral force, the quicker the vehicle will turn in the desired direction. However, the vehicle wheels will always travel at least partially in a longitudinal direction relative to the traveling direction of the vehicle. The wheel slip angle is the difference between the steering direction of the vehicle and the actual direction of the vehicle. A small amount of wheel turning allows the vehicle to go in the intended direction more quickly because the lateral force is the greatest. The lateral force is reduced if the vehicle steer angle is greater than a certain amount relative to the travel direction of the vehicle, depending on road conditions, type of tire, etc.
A vehicle wheels longitudinal slip is the amount of slip the wheel has relative to the roadway in the direction that the vehicle is traveling. The greater the longitudinal slip, the less the lateral force is available to turn the vehicle. The longitudinal slip is defined as the ratio between the wheel rotation speed and the vehicle speed.
Known rear-wheel steering assist systems are based on the assumption that there is no longitudinal slip in the rear wheels, and thus the lateral force is at a maximum. In the known systems, if the longitudinal slip reduced the ability of the wheels to provide the intended yaw rate, additional gain was added to increase the rear wheel steering assist, which would reduce the lateral force available to provide the steering.
A closed-loop control system for rear-wheel steering is disclosed in U.S. patent application Ser. No.10/305,378, filed Nov. 26, 2002, titled “Method and Apparatus for Vehicle Stability Enhancement System,” assigned to the assignee of this application and herein incorporated by reference. It was recognized in the '378 application that when the vehicle is operated in a two-wheel drive (2WD) mode or a four-wheel drive (4WD) mode, the wheel capability at the rear wheels is different because of the different amount of longitudinal force exerted on the wheel. This affects the lateral force capability, and thus, the '378 control provides control gains dependent on the selection of vehicle operation of 2WD and 4WD to result in a best over all vehicle performance.
Even though the tire force capability for 2WD and 4WD is different statistically, a control process optimized based on the selection of such operation still has room for improvement due to the fact that the precise amount of longitudinal force, and subsequently, the lateral force capability, exerted at the wheel is not only determined by the 2WD/4WD gear selection, but by the driver's operation of the accelerator pedal and the brake pedal. It would be beneficial to provide a closed-loop rear-wheel steering control system that utilized longitudinal wheel force to determine the proper amount of control gain for the rear-wheel steering to reflect the need of a different steering control under various situations of vehicle operation.